The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
D2D communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite such as WiFi Direct. These systems operate in unlicensed spectrum.
Recently, D2D communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Typically, it is suggested that such device-to-device communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes is a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the device-to-device services and cellular services is more flexible and provides higher spectrum efficiency.
The ProSe Study Item recommends also support D2D operation between out of NW coverage user equipments (UEs) and between in-coverage and out-of-coverage UEs. In such a case, certain UEs may regularly transmit synchronization signals (for example, Device to device Synchronization Signal (D2DSS)) and provide local synchronization to their neighbor UEs. The ProSe Study Item recommends also support for inter-cell D2D scenarios where UEs camping on possibly unsynchronized cells are able to synchronize to each other.
It is also agreed in 3GPP in the ProSe SI in Long Term Evolution (LTE) that D2D capable UEs will operate D2D within the UL spectrum (for Frequency Division Duplexing (FDD) spectrum) and UL subframes (for Time Division Duplexing (TDD) spectrum). Therefore, D2D UEs are not expected to transmit sync signals in DL spectrum, differently from eNBs.
eNBs provide synchronization by periodically transmitting sync signals (for example, Primary Sync signal (PSS)/Secondary Sync signal (SSS)). Such signals are also intended for cell search operation and for acquiring initial synchronization. PSS/SSS are generated based on pre-defined sequences with good correlation properties, in order to limit inter-cell interference, minimize cell identification errors and obtain reliable synchronization. In total, 504 combinations of PSS/SSS sequences are defined in LTE and are mapped to as many cell IDs. UEs that successfully detect and identify a sync signal are thus able to identify the corresponding cell-ID, too. FIG. 1 illustrates PSS and SSS time positions in case of FDD and TDD, and FIGS. 2 and 3 illustrate generation and structure of PSS and SSS.
D2D requires UEs to be able to synchronize to each other directly in order to support direct communication. It has been discussed in 3GPP that the legacy LTE sequences may be considered for sync signals (D2DSS) transmitted by UEs:
Working Assumption:
Synchronization sources transmit at least a D2DSS: D2D Synchronization Signal                May be used by D2D UEs at least to derive time/frequency        May (FFS) also carry the identity and/or type of the synchronization source(s)        Comprises at least a PD2DSS                    PD2DSS is a ZC sequence            Length FFS                        May also comprise a SD2DSS                    SD2DSS is an M sequence            Length FFS                        
Even though a range of different distributed synchronization protocols are possible, one option that is being considered in 3GPP is based on hierarchical synchronization with the possibility of multihop sync-relay. In short, some nodes adopt the role of synchronization masters (sometimes referred to as SH, Synchronization head, or CH, Cluster Head) according to a distributed synchronization algorithm. If the synchronization master is a UE, it provides synchronization by transmitting D2DSS and/or PD2DSCH (Physical Device to Device Synchronization Channel). If the synchronization master is an eNB it provides synchronization by PSS/SSS and broadcast control information by, e.g., MIB/SIB signaling. The synchronization master is a special case of synchronization source that acts as an independent synchronization source, i.e., it does not inherit synchronization from other nodes by use of the radio interface.
UEs that are under coverage of a synchronization source may, according to predefined rules, transmit D2DSS and/or PD2DSCH themselves, according to the synchronization reference received from their synchronization source. They may also transmit at least parts of the control information received from the synchronization master by use of D2DSS and/or PD2DSCH. Such mode of operation is here termed as sync-relay or control plane (CP)-relay.
In order to limit error propagation and limit dependency on a single failure point, it has been proposed to limit the number of CP-relay hops to a predefined number. The hop numbers are counted from the synchronization master.
There are a number of issues associated with multihop synchronization. For example, the receiver needs to assess the hop number associated to a certain synchronization signal because the hop number contributes to the distributed synchronization protocol (e.g., sync sources with low hop-number are preferred as synchronization references). However, if D2DSS is generated according to the 3GPP working assumption, it is impossible for the receiver to identify the associated hop number. In addition, considering that a given CP-relay UE may be only aware of D2DSS/PD2DSCH associated with some but not all of the supported hop numbers, interference towards other D2DSS not detected by the UE may be generated.
In view of the foregoing problems, it would be desirable to identity the associated hop number, to efficiently transmit the D2D synchronization signals.